The present invention relates to ultrasonic testing of high temperature materials, such as metals, ceramics and composite fabrications, in order to detect internal flaws. In particular, the present invention relates to the combination of a buffer and a couplant in an ultrasonic testing device which can detect flaws in steel at temperatures in the range from about 1700.degree. F. to about 2200.degree. F.
In the reheating method of converting steel from ingot to bloom or slab form after stripping, the ingots are held in the soaking pits of the blooming or slabbing mill, wherein they are brought to a uniform temperature of about 2400.degree. F. The ingots are then removed from the soaking pit and placed on the entry roll table of the rolling mill. The ingots are passed along the roller table to the reducing stand which shapes the ingots into the blooms or slabs. The bloom or slab workpiece is advanced toward a cutting device, such as a crop shear or a cutting torch, at the end of the mill roller table. The cutting device severs the product to the designated length by cropping sufficient scrap from the two ends of the product to correspond to what was the top and the bottom of the original ingot. This cropping of the workpiece insures the elimination of primary pipe, mechanical foldover or fish tail, porosity, and other similar defects.
In the continuous casting of steel, changes can occur in the internal quality of the cast steel because of changes made in the continuous casting process to compensate for variations which may occur in the thermal and metallurgical characteristics of the process. Anomalies such as subsurface cracks, internal cleanliness and/or center looseness or porosity must be detected prior to shipment of the product. It is well recognized that with a suitable sensor for hot in-line detection of such internal discontinuities, process control action could be initiated immediately to correct these deficiencies in quality. In addition, a computer based system could immediately readjust cut length at the runout in order to compensate for steel which has been removed by the cropping shear or cutting torch because of poor internal quality.
The problem of accurately determining the optimum cropping point in the hot steel products has plagued the steel industry for years. The location of this optimum cropping point is in the sound product just beyond the extent of the flaw. Traditionally, locating the cropping point has been dependent upon the experience and judgment of the cropping shear operator. He would often have to shear the product more than once before he would cut through sound product. This procedure resulted in a waste of the operator's time as well as in a reduction of product yield.
It has been estimated that the capability of effectively exercising process control to rapidly compensate for internal quality changes sensed in-line would represent, conservatively, an improvement of 1% in yield. In a typical steel producing installation, such an improvement could represent an annual savings of 1.0 to 1.4 million dollars per year.
Ultrasonic transducers are commercially available which can scan a relatively cold slab, bloom or billet and reveal any internal nonhomogeneous portions of the product, since an acoustic couple can be easily achieved with the cold steel. The use of ultrasonics in testing steel products at relatively high temperatures is a different matter, however. Many investigators have tried ultrasonics but have encountered problems in attempting to achieve a proper acoustic couple between the workpiece and the ultrasonic transducer. Normal production temperatures of about 1950.degree. F. are detrimental to transducers commercially available and, therefore, proper intimate contact between the product and the transducer to form the acoustic couple is very difficult. Without the necessary acoustic couple, an accurate determination of any flaw in the workpiece is not possible.
In one device for severing hot steel, an ultrasonic transducer is fitted with a water jacket and then embedded in one of the blades of a crop shear. When the shear blade contacts the hot workpiece with sufficient pressure, an acoustic couple is formed between the transducer and the workpiece through the shear blade. Before the cut is made, the sheer operator is able to ultrasonically scan the hot workpiece to determine the optimum cropping length for the elimination of flaws from the workpiece. This system is disclosed in U.S. Pat. No. 4,175 442 to Terry.
A more recent device which is capable of detecting flaws within the hot steel workpiece by ultrasonic testing was demonstrated at Argonne National Laboratory in June of 1986. This device is the result of funding by the American Iron And Steel Institute and a cooperative technical effort between Magnaflux Corporation (Chicago, Illinois), Argonne National Laboratory (Argonne, Illinois) and the National Bureau of Standards (Gaithersburg, Maryland). The device is called a Rolling Contact Ultrasonic Transducer, and it can detect flaws as small as 1/4 inch in steel at temperatures of about 2000.degree. F. The ultrasonic transducer which generates the sound waves is placed in contact with the inside rim of a thermal buffer comprising a large stainless steel wheel having a diameter of three feet. With a cooling system, the transducer is maintained cool enough to carry out an inspection of the hot steel. The buffer wheel is rolled by means of computer control over the hot slab or billet to be tested. Sound waves travel through the wheel rim and into the hot steel directly by high pressure contact between the wheel and the billet. The contact between the wheel and the billet is enhanced to an elevated pressure by a hydraulic mechanism which presses the wheel tightly down upon the surface of the hot steel billet. Sound waves reflecting off of defects within the hot steel can be observed on the monitor of a small computer. A color defect map of the steel can be generated by the computer and this color map clearly defines the bad region of the specimen which must then be severed from the billet. This device was disclosed in a paper delivered at the University of California, San Diego, on Aug. 3-8, 1986, during a conference entitled "Review Of Progress in Nondestructive Evaluation". The paper was entitled "Advances In The Development Of An In-Line Sensor System For The Internal Inspection Of Hot Steel".
With this then being the state of the art, it is an object of the present invention to provide an improved apparatus for detecting internal flaws within hot materials, such as metals, ceramics, and composite fabrications such as laminates or metal matrices.
It is another object of the present invention to provide an improved apparatus for detecting internal flaws within hot materials wherein no cooling system is required for the detection apparatus.
It is a further object of the present invention to provide an improved apparatus for detecting internal flaws within hot materials wherein no hydraulic system is required to push the detector tightly against the workpiece in order to obtain a good couple for transmitting sound into the workpiece being tested.
These and other objects of the present invention, as well as the advantages thereof, will become more clear from the description which follows.